The present invention generally relates to ion exchange resins comprising granules of a crosslinked polymer comprising an xcex1,xcex2-unsaturated acid, like poly(acrylic acid), or a vinyl monomer containing an amino group, like poly(vinylamine). The ion exchange resins can be used, for example, in the purification of water, sugar refining, recovery of transition metals, recovery of proteins from fermentation broths and agricultural by-products, and in pharmaceutical separations technology.
Ion exchange generally is defined as a reversible chemical interaction between a solid and a fluid, wherein selected ions are interchanged between the solid and fluid. An exemplary ion exchange process includes a cation exchange process wherein a fluid passes through a bed of porous resin beads having charged mobile cations, such as hydrogen or hydroxide ions, which are available for exchange with metal ions or anions present in the fluid. The ion exchange resin readily exchanges hydrogen ions for the metal ions, or hydroxide ions for other anions, present in the fluid as the fluid passes through the bed.
In time, the number of hydrogen or hydroxide ions available for exchange with metal ions or other anions diminishes. Eventually, the resin becomes exhausted and cannot perform any further ion exchange (i.e., all available exchange sites are occupied). However, the resin can be regenerated. Regeneration is accomplished using a regenerant solution, which, in the case of a cation exchange resin, comprises an acid, i.e., a large excess of hydrogen ions, that is passed over the ion exchange beads and drives the collected ions from the resin, thereby converting the ion exchange resin back to its original form.
A specific example of a cation exchange process is the purification/softening of tap water. In this process, weak acid ion exchange resins use carboxyl radicals, in the sodium form, as the cation exchange site. The sodium ions are the charged mobile cations. Alkaline earth metals, such as calcium and magnesium, present in the tap water are exchanged for the sodium cations of the resin as the water passes through a bed of the ion exchange resin beads. Removal of calcium and magnesium ions from water in exchange for sodium ions via weak acid cation exchange resins is not limited to the water purification/softening applications, but also includes the softening of fluids, such as clay suspensions, sugar syrups, and blood, thereby rendering the fluids more amenable to further processing. When the exchange capabilities of the ion exchange resin are exhausted, a weak acid can be used to regenerate the acid form of the resin, followed by conversion of the acid form of the resin to the sodium form with dilute sodium hydroxide.
Similarly, an anion exchange resin containing anionic radicals removes anions, like nitrate and sulfate, from solution. Anion exchange resins also can be regenerated with a sodium hydroxide solution, for example.
The reversibility of the ion exchange process permits repeated and extended use of an ion exchange resin before replacement of the resin is necessary. The useful life of an ion exchange resin is related to several factors including, but not limited to, the amount of swelling and shrinkage experienced during the ion exchange and regeneration processes, and the amount of oxidizers present in a fluid passed through the resin bed.
Cation exchange resins typically are highly crosslinked polymers containing carboxylic, phenolic, phosphonic, and/or sulfonic groups, and roughly an equivalent amount of mobile exchangeable cations. Anion exchange resins are similar but contain amino groups and roughly an equivalent amount of mobile exchangeable anions. Suitable ion exchange resins, preferably, (a) possess a sufficient degree of crosslinking to render the resin insoluble and low swelling; (b) possess sufficient hydrophilicity to permit diffusion of ions throughout its structure; (c) contain sufficient accessible mobile cation or anion exchange groups; (d) are chemically stable and resist degradation during normal use; and (e) are more dense than water when swollen.
Hatch U.S. Pat. No. 3,957,698 discloses production of weak acid ion exchange resins by the copolymerization of methacrylic or acrylic acid, in a low molecular weight hydrocarbon diluent, with 0.5 to 10 wt. % of divinylbenzene, based on the weight of initial monomers, to achieve the proper degree of crosslinking. In order to prepare high purity ion exchange resins by this process, the resin is heated at a high temperature or is extensively washed with a solvent to remove the diluent. The resin particles are in the size range of 2 to 5 microns.
Meitzner et al. U.S. Pat. No. 4,224,415 discloses the preparation of ion exchange particles prepared by suspension copolymerization of water-insoluble monomers, such as methyl acrylate and methyl methacrylate, with a crosslinking agent such as divinylbenzene. In addition, a precipitant is added to the monomer phase to impart a reticular nature to the resulting particles. This process requires that the divinylbenzene content be in the range from 8 to 25 wt. %. These particles must be hydrolyzed with a strong base in order to prepare a resin with exchangeable functionalities.
Therefore, conventional weak acid cation exchange resins typically are produced using a multi-step process. The first step is a batch, aqueous suspension polymerization of methyl acrylate monomer, in the presence of divinylbenzene, to provide crosslinked beads of methyl acrylate. The poly(methyl acrylate) beads then are reacted with sodium hydroxide to hydrolyze the ester groups of the poly(methyl acrylate), and thereby introduce carboxylate (i.e., weak acid) functionality into the beads. Due to solubility of acrylic acid in the aqueous phase of the suspension polymerization, acrylic acid is not wholly substituted for the methyl acrylate monomer in the foregoing process. Therefore, conventional manufacturing processes preferably utilize methyl acrylate, which is a relatively expensive monomer, in the syntheses of the ion exchange resin. Acrylic acid, however, can be copolymerized in a batch process with methyl acrylate monomer utilizing a divinylbenzene crosslinker.
In view of the foregoing, it would be desirable to provide an ion exchange resin, either cation or anion, produced from less expensive monomers such as, for example, acrylic acid. Furthermore, it would be desirable to provide an ion exchange resin that can be manufactured in a simple, continuous process, and that exhibits the excellent ion exchange properties and physical properties of prior ion exchange resins, or improves upon these properties. Additionally, it would be desirable to minimize the amount of organic solvents used in the preparation of an ion exchange resin.
The present invention is directed to ion exchange resins comprising a polymerized and crosslinked xcex1,xcex2-unsaturated acid, or a polymerized and crosslinked vinyl monomer containing an amino group, and to methods of manufacturing the same. More particularly, the present invention is directed to ion exchange resin granules comprising a crosslinked polymer or copolymer comprising an xcex1,xcex2-unsaturated acid, like acrylic acid, or a vinyl monomer containing an amino group, like vinyl amine, that preferably have a highly crosslinked surface.
One aspect of the present invention, therefore, is to provide an ion exchange resin comprising a dry, granulated, polymerization product comprising: (a) an xcex1,xcex2-unsaturated acid monomer, or a vinyl monomer containing an amino group, or salt thereof, (b) optional vinyl monomers, (c) a bulk crosslinking agent, and (d) a latent crosslinking agent and/or a surface crosslinking agent. In preferred embodiments, the granulated polymerization product is crosslinked with a latent and a surface crosslinking agent. In most preferred embodiments, the polymerization product contains a latent crosslinking agent.
The bulk crosslinking agent, which provides internal crosslinks between the polymer chains, has at least two polymerizable carbonxe2x80x94carbon double bonds. The latent crosslinking agent, which also provides internal crosslinks through the pendant acid or amino groups of a polymer includes (a) compounds having at least one polymerizable double bond and at least one functional group reactive with an acid or an amino group, like a carboxylic acid group, (b) compounds having at least two functional groups reactive with acid or amino groups, (c) polyvalent metal compounds capable of forming ionic crosslinks with acid groups, and (d) mixtures thereof. The surface crosslinking agent typically is a diglycidyl ether, a polyhydroxy compound, a hydroxyalkylamide, an alkylene carbonate, or a mixture thereof.
Another aspect of the present invention is to provide a method, preferably a continuous method, of manufacturing an ion exchange resin that includes the steps of polymerizing an aqueous solution comprising an xcex1,xcex2-unsaturated acid, like acrylic acid, or a vinyl monomer containing an amino group, like vinyl amine, or a salt thereof, optional vinyl monomers, a bulk crosslinking agent, and optionally a latent crosslinking agent to form a hydrogel of the polymerized monomer. Optionally, subsequent processing steps can be used to add a latent crosslinking agent. The resulting hydrogel then is dried, cured, and sized to provide dried granules, followed by optionally crosslinking surfaces of the granules with a surface crosslinking agent to form the ion exchange resin. In each case, the ion exchange granules contain a bulk crosslinking agent, and a latent crosslinking agent and/or a surface crosslinking agent, in a total amount of crosslinking agents such that the granules absorb less than about 25 times, and preferably less than about 15 times, their weight in tap water under no load (i.e., AUNL), and resist swelling to a sufficient degree to perform as an ion exchange resin.
Yet another aspect of the present invention is to provide a method of manufacturing, either batchwise or continuously, an ion exchange resin comprising polymerizing one or more monounsaturated acid monomers, like acrylic acid, one or more vinyl monomers containing an amino group, like vinyl amine, or salts thereof, and optional vinyl monomers, in the presence of about 0.01 to about 3 mole %, based on the total number of moles of the monomers, of a polyvinyl bulk crosslinking agent, to provide a bulk crosslinked polymer hydrogel, followed by the addition of about 0 to about 6 mole %, based on the total number of moles of the monomers, of a latent crosslinker to further crosslink the bulk crosslinked hydrogel, then heating the hydrogel-latent crosslinking agent mixture at a sufficient temperature for a sufficient time to dry and cure the hydrogel and to form crosslinks through pendant acid or amino groups present on the polymer chains, thereby generating ion exchange resin particles. Optionally, the ion exchange resin particles then can be surface crosslinked with 0 to about 2 weight % of a surface crosslinking agent, based on the weight of the particles.
Alternatively, the latent crosslinking agent is added to the mixture of monomers and polyvinyl bulk crosslinking agent. The latent crosslinking agent does not react during the polymerization reaction, but is available for subsequent reaction with pendant acid or amino groups on the polymer during a heating step.
The step of surface crosslinking can be achieved by coating surfaces of the ion exchange granules with a solution containing a surface crosslinking agent, and then heating the coated granules at a sufficient temperature and for a sufficient time to dry and cure the granules and provide surface crosslinks. In an embodiment wherein a latent crosslinking agent is omitted, a surface crosslinking agent is required to provide the degree of crosslinking needed for an ion exchange resin. Similarly, if a surface crosslinking agent is omitted in an embodiment, a latent crosslinking agent is required to provide sufficient crosslinking for an ion exchange resin. A bulk crosslinking agent is utilized in all embodiments of the ion exchange resin of the present invention.
A present ion exchange resin is in the form of irregular granules, as opposed to spheres. The granular form of the ion exchange resin provides a greater surface area than spheres, thereby providing a more efficient ion exchange. In some embodiments, the present ion exchange resin granules generally have a relatively xe2x80x9csoftxe2x80x9d interior, i.e., the degree of internal crosslinking (i.e., bulk plus latent crosslinking) is sufficient to resist granule swelling and to allow percolation of a liquid through the particles. The granules, however, typically have a relatively xe2x80x9chardxe2x80x9d surface, i.e., the degree of surface of surface crosslinking is high, i.e., at least 1000 ppm, and preferably about 1000 to about 20,000 ppm, to impart structural integrity to the granules and to further prevent swelling of the granules during use.
Further aspects and advantages of the invention will become apparent from the following detailed description of the preferred embodiments of the invention, taken in conjunction with the examples and the appended claims.